Bohr's discovery of the quantum nature of the atom, published when he was a young man of 28, was an important pioneering contribution to the earliest days of quantum physics.

This field emerged to explain the common sense-defying behaviour of atoms, molecules and light at the smallest scales, forming the foundations on which we have built one of the greatest and most successful theories of all time — quantum mechanics.

What is quite remarkable to modern eyes was that Bohr had very little to go on.

The true nature of the atom as an incredibly tiny nucleus surrounded by a cloud of orbiting electrons had only been discovered a few years earlier, in the separate work of physicists Thomson and Rutherford.

Bohr's genius was to recognise that these electrons had many roles in a range of apparently different scenarios. He saw that electrons were behind the electric currents flowing in wires, the red hot glow of molten iron, and the production of light from electric discharges in gas-filled tubes.

Bohr took the important elements of the emerging theories to explain all these different things, invented some new quantum mechanical principles and made it all work.

In so doing he also managed to solve an important and troubling problem: that any electron moving in an orbit would have to spontaneously radiate away energy until it spiralled down and slowed to a stop — a view from classical physics that meant no atom could be stable.

Bohr's quantum atom: nature is digital

Like others, Bohr was keen to draw on our understanding of the orbit of planets around the sun in understanding the orbit of electrons in atoms.

The planets are attracted by the powerful gravity of the sun, but their speed lets them settle into stable orbits rather than spiralling into the sun's gravitational field.

In the case of the positively charged nucleus and the negatively charged electron, the mutual pull is the electric force. Classical physics dictates that an accelerating charge (like an electron in orbit) must give off electromagnetic radiation. The energy lost through radiation should make an electron slow in its orbit and quickly crash into the nucleus, which means no atom could be stable. This was clearly not true, and Bohr's solution to this conundrum was the first of two powerful ideas with which he introduced us to the quantum atom.

He proposed that electrons in atoms are only stable in certain allowed orbits, which he called stationary states. This idea is an attribute of the wave-like nature of all matter at the nanoscale, and it is now understood as a fundamental principle of quantum mechanics.

Bohr's second idea was that electrons dropping down from one stable orbit to another would radiate a single discrete packet of radiation, in the form of a photon of light. This shows the deep connection between light and matter, and that photons are all or nothing — there is no such thing as half a photon. Together these ideas tell us that nature is fundamentally digital at the atomic level, and they provide the basis for quantum mechanics.

From theory to evidence

Bohr was able to use his new theory to successfully explain the regularities in the pattern of light emitted from hot hydrogen gas, both in the laboratory and in the atmospheres of stars near and far. Heated hydrogen emits characteristic blue, red and violet light. Bohr showed that the light was given off by excited electrons as they settle into allowed stable orbits at lower energies. A photon of each of those colours of light corresponds to the energy difference between different allowed orbits.

The radiation emerging from the atom as the electrons settle into stable orbits can tell us a lot about the nucleus. Shortly after Bohr's discovery, Henry Moseley discovered that energetic photons emitted from electrons settling into close orbits around the nucleus, typically in the x-ray part of the spectrum, could be used to discover gaps in the periodic table of the elements where new elements would later be found. Later, Bohr's theory was further developed to explain molecules and the basis of chemistry.

One year after Bohr's theory appeared in the scientific journals, the British Association for the Advancement of Science held its 1914 meeting in Australia. In the old physics building at the University of Melbourne, Sir Ernest Rutherford presented a report on the new and controversial theory to delegates from the United Kingdom, Australia and New Zealand.

This was one of the first public outings for the theory. Reports from the conference give a strong sense of the excitement created by Bohr's radical ideas — one delegate remarked "... I should like to say that although I have criticised certain parts of Bohr's theory adversely, no one can admire more its ingenuity and great suggestiveness." These were prophetic words!

Today, Bohr's theory is applied to a range of scenarios that would have astounded the young Niels.

He could never have imagined that his work would lead to PET (positron emission tomography) scanners that look inside our bodies, showing us the effect of diseases like cancer on the way our organs function. Bohr's theory explains the mutual orbit of electrons and positrons just before they annihilate each other, transforming into gamma rays that give rise to the PET scan image.

And recent breakthroughs have led to some exciting new applications built on Bohr's theory, including our work in nanodiamonds and quantum computing at the Australian Centre of Excellence for Quantum Computation and Communication Technology.

Bohr in today's science: nanodiamonds, quantum computers ...

Bohr's theory can be adapted to explain the peculiar orbits of electrons around a single nitrogen atom inserted into a diamond crystal. The light photons emitted when these electrons change between their stationary states is incredibly bright, and signals the internal quantum state even at room temperature. These stationary states are susceptible to even the tiniest magnetic fields, affecting the colour of light given off. When a living cell is seeded with nanoscale diamond crystals containing single nitrogen atoms, the way the cellular electromagnetic machinery affects the emitted light tells us what is going on at these tiny scales. This could help us learn about the dynamics of biological neural networks, which is fundamental to gaining insight into information processing in the brain.

We have also shown how modern nanotechnology allows us to program digital information into the quantum atom. Recognising that both the electrons and the nucleus in the quantum atom possess angular momentum, called spin, we have discovered how to amplify one billion-fold the subtle difference in energy between the two stable spin states of the nucleus of an engineered phosphorus atom in a silicon device. This could lead to a raft of new technologies built on the quantum atom.

For example, instead of seeking information from the photons emerging from quantum atoms, we use photons in our single atom device as a means of artificially encoding information in the nuclear spin orientation. This could be the foundational component of a large-scale silicon quantum computer. In this device the electron spin is used for information processing and read-out, with the nuclear spin used as long lived memory for quantum information.

A quantum computer could have revolutionary applications to the storage, processing and transmission of information. This would exploit the best characteristics of the quantum domain and the most important material for microelectronics, silicon, to build the proposed quantum internet of the mid-21st century.

... and the Higgs boson

After Bohr published his ideas in 1913 he went on to found an important institute for theoretical physics in Copenhagen, and his great discovery was recognised by a Nobel Prize in 1922. He pledged support for founding the CERN laboratory in 1952 and then hosted the CERN theorists in his institute until they were ready to move to Geneva. Australian involvement in CERN led to the announcement in 2012 in Melbourne and Geneva of the discovery of the Higgs boson, the latest discovery in the deep journey into the quantum atom that Bohr helped start one hundred years ago!

Comments (6)

Comments for this story are now closed. If you would like to have your say on this story, please email ABC Science

James :

18 Jul 2013 9:59:34am

The idea of a working quantum processor within 50 years is an exciting notion. Thank you for this interesting and informative article, I am taking physics for my next senior elective course, I cannot wait to learn more about quantum mechanics, general and special relativity and many other interesting atomic theoriies.

Celia Berrell :

Francis Higgins :

19 Jul 2013 5:22:30pm

Einstein was deeply unhappy with Quantum theory. Quantum theory is the derivation of Wave Mechanics and too little consideration is given to this. That light refracts as it passes the Sun, then the Matter Wave refracts also. Time dilation across the Wave function of light being in line with Einstein's vision of Gravity.

Asher Kirschbaum :

26 Jul 2013 12:59:51pm

Quantum Mechanics (QM) is the commonly accepted theory today, but that doesn't mean it's successful. The theory describes a world made of particles, but there are numerous problems with the particle picture. For example, it's just not possible for electrons in an atom to be particles, as shown in the book, “Fields of Color: The theory that escaped Einstein”: “According to Maxwell’s equations, an electron moving in an orbit must radiate energy in the form of EM waves, and as it loses energy it will move closer to the nucleus, just as an orbiting spacecraft returns to earth by firing retro-rockets… The energy lost by EM radiation from an orbiting electron would cause it to spiral inward and eventually crash into the nucleus. But that doesn’t happen. Conclusion: Electrons can’t be particles in orbit." And that's just one of the problems

Quantum Field Theory (QFT) replaces particles with fields. In QFT electrons are fields surrounding the nucleus, not particles in orbit. For more information, visit http://www.quantum-field-theory.net/fields-of-color or google “Einstein’s enigmas.”

whatdoctor :

15 Aug 2013 11:28:55am

This article touches on electrons orbiting creating an electric field that would radiate energy away. it does not explain why this does not happen in the fixed orbits. Is it that the electron is really just a field around the nucleus or that the energy that would be given off is less than one quantum amount?